With developments in semiconductor manufacturing process technology many limitations in miniaturization and integration of memory devices and non-memory devices exist. Particularly, as the design rule for semiconductor devices rapidly reduces, punch-through phenomenon between a source and a drain is frequently generated to cause a great limitation in operation characteristics and performance characteristics of the semiconductor devices.
In detail, in the case of flash memory miniaturization is particularly in rapid progress. Reduction in an effective channel length of a flash memory device improves not only integration degree but also performance.
That is, when the effective channel length reduces in the flash memory device, a program operating speed is remarkably improved due to an increase in efficiency of carriers injected into a floating gate forming a flash memory cell.
However, when the effective channel length is reduced in the flash memory device, a drain depletion region extends to merge with a depletion region of a source. Accordingly, a punch-through phenomenon between the source and the drain may occur.
Consequently, when the effective channel length is reduced in a flash memory device, a threshold voltage of a cell greatly reduces and has a great influence on the operation characteristic of the flash memory cell.